This invention relates to a process for recovering lignin from a solution of lignin in a water miscible organic solvent such as a lower aliphatic alcohol. This invention particularly relates to a process for recovering lignin from the alcohol/water extract or "black liquor" produced as a by-product of the pulping of wood or other fibrous plant material with an alcohol and water solvent at elevated temperatures and pressures to produce a cellulose pulp. This invention also relates to a lignin product produced by the process.
Processes for treating wood with organic solvents, such as alcohols, to separate the wood's lignin, hemicellulose, sugar and cellulose fractions are now well known. See, for example, Kleinert et al U.S. Pat. No. 1,856,567 and Kleinert U.S. Pat. No. 3,585,104. Such solvent pulping processes have appeared to be attractive alternatives to conventional chemical pulping processes, such as kraft and sulfite pulping processes, which suffer from relatively high equipment costs and pollution problems.
One solvent pulping process, disclosed in Diebold et al U.S. Pat. No. 4,100,016, has appeared to be particularly attractive in providing highly efficient recovery of its alcohol solvent, separation of the cellulose and lignin fractions of wood, and recovery of cellulose pulp with no appreciable air or water pollution or solid waste products. This patented process has also provided hardwood pulps with yields, strengths, Kappa numbers, viscosities, fiber strengths and bleachability characteristics that are equal to or better than kraft and sulfite hardwood pulps.
However, the recovery of lignin from the alcohol/water black liquor, generated by the solvent pulping process of Diebold et al U.S. Pat. No. 4,100,016, has been relatively inefficient and difficult to control. Lignin has been recovered from the black liquor in this patent by first stripping (preferably vacuum stripping) alcohol from the black liquor and then separating the lignin which precipitates from the stripper bottoms or tails (preferably by thickening and then centrifuging the settled solids from the stripper bottoms). However, a portion of the lignin has tended to precipitate as a sticky tar or gum on the internal surfaces of the stripper, thereby fouling the stripper and reducing its efficiency in recovering alcohol from the black liquor. The lignin also has tended to precipitate from the stripper bottoms as a sticky amorphous mass which has been difficult to handle and has required substantial crushing to convert the lignin mass into a powder.
As a result, more efficient ways have been sought for removing lignin from the black liquor produced by a solvent pulping process such as is disclosed in Diebold et al U.S. Pat. No. 4,100,016. One method has involved precipitating lignin from the alcohol/water black liquor by diluting it with water. See Rydholm, "Pulping Processes", pp. 672-673, Interscience Publishers, New York (1971). However, this method has resulted in very slow settling rates of the lignin, and in some cases, a very stable colloidal suspension of the lignin has been formed which has been difficult to filter or centrifuge. There has been a continuing need, therefore, for a relatively simple way of recovering lignin from an alcohol/water black liquor in high yields and at high rates in an easy to handle and useful form.